Added documentation for In-App Payments API.

chrome/common/extensions/docs/static/css/site.css

Index: chrome/common/extensions/docs/static/css/site.css
diff --git a/chrome/common/extensions/docs/static/css/site.css b/chrome/common/extensions/docs/static/css/site.css
index f7591a0f00c1553aec93d5d5b3341b6dbd05d250..12784967bc0a7f2fd7d96b51066b0d13d220d30a 100644
--- a/chrome/common/extensions/docs/static/css/site.css
+++ b/chrome/common/extensions/docs/static/css/site.css
@@ -566,6 +566,12 @@ button.google-button:active {
   clear: right;
 }
 
+/* small indent for better visual distinction
+   (e.g., in a long list) */
+.indent-small {
+  margin-left: 2em;
+}
+
 /* Tabbed pane with header (tabs) and content */
 tabs {
   margin: 25px 0;